In a military environment, it is essential to determine whether an approaching aircraft or other vehicle is a friend or an enemy. This determination is generally accomplished by decision making apparatus called Identification-Friend-or-Foe (IFF) systems. In a typical IFF system an interrogating station transmits an interrogation signal to which a transponder in the approaching vehicle automatically replies with an answer or reply-signal. The interrogation and reply signals are generally coded in a cryptographic fashion according to needs of the situation and a correct reply to a given interrogation is one that is coded in a predetermined manner.
Virtually every IFF system has as its fundamental element a Friend-Accept decider, also commonly known as a "Reply Evaluator." It is this Friend-Accept decider which determines whether a sufficient number of received correct reply-signals come from a friend or an enemy. An enemy may be attempting to guess the code, or repeat previously intercepted "friend" replies, or attempt to interfere with reception of replies transmitted by friends. Hopefully, a Friend-Accept decider will be able to detect these attempts and initiate appropriate action.
Many varieties of IFF systems are currently in use. Present day systems generally allow the enemy too much of a chance to be accepted as a friend, or are too vulnerable to disruptive interference or jamming (which can effectively deny acceptance by an IFF system of a true friend as a friend). One type of Friend-Accept decider utilizes a Friend-Accept decision criteria that accepts an approaching vehicle as a friend only when it responds with a series of correct replies to a predetermined number of interrogations. In other words, it examines a received set of responses for a "run" of a predetermined number of correct replies in succession.
One of the significant disadvantages of many previous systems is that an enemy, by simply increasing the reply signal power with respect to a "friend" (located the same distance from the IFF antenna) could increase the number of chances to transmit a proper reply signal (because the greater signal energy will then be received over a larger angular antenna scanning space as the antenna sweeps by the target vehicle). Also, in many previous systems, the enemy can deny a friendly vehicle acceptance as a friend by simply transmitting periodic jamming signals which interfere with reception of correct reply signals. This is because a run of correct replies could be interrupted by a jamming signal, and the system would then begin to look for a new set of correct replies.
Some other types of IFF systems utilize a sliding window technique in which an essentially unlimited number of samples are used in carrying out a continuous evaluation process to determine whether an unknown transponder is a friend or foe. The continuous sliding window evaluation process examines a predetermined number of the most recently received reply signals and determines the ratio of correct to incorrect replies. This process, too, is undesirable because it allows the enemy far too many chances to come up with a proper set of reply responses. Sliding window techniques generally require a higher threshold of acceptance for a given level of decision confidence than does a sliding window technique in which the total decision sample is bounded.
Still other techniques are available which may avoid some of the deficiencies described above, but they generally require a memory capability. The present invention does not.
The present invention makes use of the well known "Sequential Observer" technique to achieve, with a simpler device, the same or improved result as that achievable by more complicated devices. Although the sequential analysis technique itself was throughly described by A. Wald in 1947 ("Sequential Analysis" John Wiley and Sons, N.Y. 1947), it has not been previously applied in the manner of the present invention.